The present invention relates to high resolution optical lithographic systems, and, more particularly, those used in the fabrication of integrated circuits.
The fabrication of very high density integrated circuits ("IC's") is dependent upon the availability of high resolution lithographic systems capable of resolving and exposing the narrow circuit linewidths onto the photoresist masks used during the manufacturing process. At present, there are several different designs for optical lithographic systems capable of producing a minimum linewidth resolution of about one (1) micron, i.e., one micrometer. Currently, the IC industry is seeking a production worthy means of exposing 0.5 micron linewidths to reduce electronic component size as required by high technology applications.
One optical lithographic design utilizes a reflective optical (1:1 projection ratio) projection system to image a narrow arc of the desired mask image onto the circuit substrate wafer and photoresist. As the system simultaneously scans the mask image and object (wafer) planes, a two dimensional image of the mask is exposed on the photoresist coated on the circuit wafer. This system, which is designed to maintain a constant optical pathlength between mask image and wafer surface, is subject to inaccuracies caused by small variations in wafer thickness, as well as variations in flatness of the wafer, wafer holder, mask and reflective optics. These mechanical problems, coupled with the difficulty of producing accurate 1:1 masks and diffraction problems associated with imaging these narrow lines, have limited the resolution of these 1:1 lithographic printers to approximately 2.0 microns. Production linewidths of 1.25 microns are achieved in these systems by using short wavelength ultraviolet sources (330 nanometers (nm)). Ultimately, the performance of this system design is limited by accumulated mechanical positioning errors and diffraction problems.
Another optical lithographic system design utilizes a reduction ratio in the projection optics (commonly 10:1), and a precise mechanical stage controlled by a laser interferometer to replicate an enlarged mask on the photoresist and wafer surface. This system relieves mask fabrication limitations of the 1:1 system, minimizes mechanical positioning errors, and reduces the errors caused by diffraction problems. Since the field of view of this optical system is smaller than the corresponding 1:1 system, linewidths as fine as 0.75 micron are possible and have been demonstrated in the laboratory. Production linewidths between 1.0 and 1.25 microns are claimed.
As an alternative, electron-beam lithographic systems have been developed which are capable of resolving 0.1 micron linewidths. However, these systems have several disadvantages which would make them undesirable if an optical system capable of 0.5 micron resolution was available. First, the use of the electron-beam systems require that the wafer or photoresist be placed in a vacuum during exposure. As a result, the cycle time for creating each IC increases, since, for each circuit layer, the wafer must be placed in the system, the system evacuated, the vacuum brought down and the wafer exposed and then replaced in the system. Contamination becomes a problem because of the increased handling of the wafer during IC fabrication. In addition, the costs of electron-beam systems are excessive in comparison to high resolution optical lithographic systems.
It is accordingly a primary object of the present invention to provide an improved focal sensor for high resolution lithographic systems.